Low thermal expansion material such as carbon-carbon composites are important materials for aerospace applications because of their high strength and toughness at high temperature. However, their use in air is limited because of oxidation. A ceramic coating should be able to provide the required oxidation protection, but because of the very low thermal expansion coefficient of high modulus carbon-carbon composites, it is difficult to find a ceramic coating with matching thermal expansion. U.S. Pat. No. 5,275,844 dated Jan. 4, 1994, the disclosure of which is herein incorporated by reference, teaches that a pyrolytic boron nitride coating can, under the correct coating conditions, produce an adherent "crack-free" coating, on low thermal expansion material, which is resistant to oxidation up to about 800.degree. C. for an extended time period of over 20 hours and at even higher temperatures for shorter time periods. A "crack-free" coating is defined in U.S. Pat. No. 5,275,844 as being essentially free of cross-plane cracks which would otherwise allow air to penetrate to the underlying carbon-carbon composite or other low thermal expansion material. Using the process described in the aforementioned patent, a pyrolytic boron nitride coating may be formed on a carbon-carbon composite structure having a thermal expansion between 0.14 to 0.32 percent over a temperature range between room temperature and 1500.degree. C., which will closely conform to the thermal expansion of a typical carbon-carbon composite. However, a conventional pyrolytic boron nitride coating will not sustain a temperature environment exceeding 1000.degree. C.
Alternate ceramic materials such as silicon carbide have been extensively studied, but its thermal expansion to 1500.degree. C. is more than three times that of a carbon-carbon composite so that cracks form in the coating on cooling from deposition temperature. Because these coatings crack, the carbon-carbon composite may instead be treated with low-melting oxidation inhibitors which flow and fill the crack. This is not a desirable solution to the problem. The same problem exists for silicon nitride, which has more than twice the expansion of a carbon-carbon composite to 1500.degree. C.